[quote="Amanda"]TLP pretty much said it all lol. Although I would add the factor of genes playing a role. Vitani and Nuka are both the cubs of Scar and seem to have inherited his lanky and weak frame. Kovu's father, whoever he was, might have been more muscular and passed that trait on to Kovu. This is all just speculation though.

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Even though I know you're speculating, but, Nuka and Vitani aren't Scar's cubs. Neither of them referred to Scar as their father in the film, and if reliable enough, the deleted scenes have them referring to him as just "Scar".
And yes, genetics does play a factor in their build. How could I forget about that? Well, I believe all of Zira's cubs share the same father, and most siblings always look different from each other because the parents's genetic traits can have multiple ways of producing their offspring as genetics is just that complicated enough to do that.